1.Field of the Invention
The present invention relates to heat protection elements made of quasicrystalline aluminum alloys.
2.Description of the Prior Art
Heat barriers are assemblies of one or more materials intended to limit the heat transfer towards or from parts and components of fittings in many household or industrial devices. It is possible, for example, to mention the use of heat barriers in heating or cooking devices, smoothing irons where the hot part is attached to the body and near heat insulation; in automobiles, at a number of points such as the turbocompressor, the exhaust system, insulation of the cabin, and the like; and in aeronautics, for example on the rear part of compressors and jet engines.
Heat barriers are sometimes employed in isolation in the form of screening, but very frequently they are used directly in combination with the source of heat or the part to be protected for reasons of mechanical behavior. Thus, use is made of sheets of mica, of ceramic plates and the like in electrical household appliances by fitting them by screwing or adhesive bonding, or else of sheets of agglomerated glass wool which are supported by a sheet of metal. A particularly advantageous process for attaching a heat barrier to a part, in particular to a metal part, consists in depositing onto a substrate the material forming the barrier in the form of a layer of thickness determined by a thermal-spraying technique such as, for example, plasma spraying.
It is very frequently recommended to use the heat barrier, which may furthermore comprise a number of layers, in combination with other materials which are also deposited as a layer by thermal spraying. These other materials may be intended to provide the barrier with protection against external actions such as, for example, mechanical impacts, a corrosive environment and the like, or else to make it easier to bond to the underlying substrate. This point is particularly important in the case of heat barriers operating in heat cycling conditions or at high temperature. The mechanical stresses which then exist at the interface with the substrate and result from the differences in the thermal expansion coefficients of the substrate and of the deposit quickly lead to damage of the barrier by shearing, when they do not prohibit its use. To overcome this disadvantage, use is frequently made of an intermediate deposit, called bond coat, which becomes plastic in the working temperature region of the barrier, and this eliminates the stresses at the barrier interface.
The material most frequently employed in aeronautics to form heat barriers is yttriated zirconia, which withstands very high temperatures. The deposition of the zirconia is carried out by plasma spraying using a conventional technique starting with the powdered material. Zirconia has a low thermal diffusivity (.alpha.=10.sup.-6 m.sup.2 /s). However, it has a relatively high specific mass .rho., and this constitutes a disadvantage in certain applications; moreover, some of its mechanical properties, such as hardness and resistance to wear and to abrasion, are low.
Other materials are employed as heat barrier. Mention may be made of alumina, which has a specific mass lower than that of zirconia, and a diffusivity and specific heat which are higher than that of zirconia, but whose mechanical properties are not satisfactory. It is also possible to mention stainless steels and some refractory steels which offer heat insulation properties but which have a high specific mass.